This invention relates to a method for repairing a surface portion of a coated article and to a repaired coated article. More particularly, it relates to a recoated article and method for recoating an article having a high temperature alloy substrate and a coating including Al on the substrate, the coating including a diffusion zone at the substrate.
Certain components of power generating apparatus, for example a turbine engine, operate in the hot gas path of the apparatus. In the turbine section of a gas turbine engine, components are subjected to significant temperature extremes and contaminants present in combustion gases. As a result of operating in such an environment, components are subject to degradation by oxidation and/or hot corrosion. To combat environmental attack, it has been a common practice in the art to protect at least an outer surface portion of such components with an environmental resistant coating. As has been widely reported in the art, many of such coatings include Al, sometimes modified with secondary elements such as one or more of Pt, Rh, Pd, Cr, Si, Hf, Zr, and Y.
In addition to degradation during service operation, difficulties in such coatings can arise during initial manufacture. For example, unacceptable coatings have been identified on a component after heat treatment and as a result of evaluation of the quality of the coating.
Generally, such coatings, forms of which frequently are referred to as aluminide coatings, during or after application to an article surface are subjected to a heat treatment that interdiffuses elements of the coating and the substrate. For example, for slurry type coatings and some pack cementation coatings, the thermal cycle used to diffuse the aluminum into a component surface is conducted after the coating cycle. Such heat treatment forms a diffusion zone between the substrate and an outer portion of the coating. One example is application of an aluminide type coating to an outer wall portion of an air-cooled gas turbine engine component, such as a rotating blade or a stationary vane or strut. The diffusion zone becomes an integral part of the component wall, generally designed to have a particular allowable thickness range based at least in part on considerations of heat transfer and structural strength. The thickness and extent of the diffusion zone can be controlled through processing parameters such as the coating time, coating and heat treatment temperature, and aluminum activity of the coating ingredients and conditions.
Difficulties or degradation related to the coating and/or to the coated article at the manufacturing level, as well as that which occurs during engine service operation, often necessitates removal and replacement of such protective coatings as well as repair of the component itself. As used herein, the term “repair” is intended to include one or the combination of repairs of the structure of the article, as well as replacement of the coating. Such repair of the component can include operations such as welding and/or braze repairing of cracks prior to replacement of the coating. The presence of an aluminide coating and its ingredients has been found to be incompatible with and detrimental to such article repair and coating replacement operations.
One example of known removal of a diffusion aluminide coating from a surface portion of an article, in preparation for article repair and/or coating replacement, has been to remove both the aluminide coating outer portion, generally rich in Al, as well as the coating diffusion zone, generally including elements from the coating outer portion and the substrate, as well as intermetallic phases. Such removal has been accomplished by a combination of mechanical abrasion and chemical stripping that removes from the substrate the outer layer and the diffusion zone portion of the coating.
According to known methods, complete removal of the aluminide coating, including the diffusion zone, from the balance of the substrate has been conducted to provide a surface that can be repaired, such as by brazing and/or welding operations, and recoated using a range of selected coatings and coating processes. For example, the presence of certain amounts of such detrimental elements as Al and/or intermetallic phases above an acceptable amount, can affect, adversely, article repair as well as the reapplication of certain environmentally protective coatings. One reported type of such a replacement protective coating includes first electrodepositing on a surface a noble metal such as Pt and then aluminiding that surface. In some examples of that type of replacement coating, unfavorable processing reactions have been observed to result, during the application of such a coating, from the presence of undesirable amounts, for example greater than that in the substrate, of residual elements such as Al from the diffusion zone, as well as certain intermetallic phases. In addition to inhibiting repair processes, presence of amounts of such element or intermetallics, or their combination, can reduce plating adhesion and inhibit the plated metal from diffusing into the substrate. As used herein, the term “undesirable feature” is intended to mean one or more of at least one undesirable element and/or at least one undesirable intermetallic phase that can be detrimental to the repair and/or replacement coating of an article.
A necessary result of removing both the coating outer portion and the coating diffusion zone, as has been conducted in known methods, is loss of wall thickness of an article. A reduced wall thickness can approach a limit for structural strength and, in any event, can reduce the total operating life of an article by limiting its potential for subsequent repair of the article coating. In addition, for air-cooled articles including cooling air discharge openings in a wall that has had its thickness reduced, loss of airflow control can occur as a result of change in size and/or shape of the openings. Removal of the diffusion zone at the surface of an opening, such as a hole, means that the size of the opening has been enlarged.